1. Field of the Invention
The present invention relates to a method of producing a liquid crystal display device which is flat and light-weight for use in a notebook-type or palm-top type personal computer, word processor, electronic datebook or the like.
2. Description of the Invention
Conventionally, a flat and lightweight liquid crystal display device has used substrates consisting of transparent electrodes formed on glass substrates.
In recent years, however, there has been a rising demand for a flatter and lighter-weight liquid crystal display device for use in a notebook-type or palm-top type personal computer, word processor, electronic datebook or the like. In order to meet the demand, a liquid crystal display device which uses substrates consisting of transparent electrodes formed on plastic substrates has been proposed.
However, the foregoing conventional device which uses the substrates consisting of the transparent electrodes formed on the plastic substrates is disadvantageous in that the gas adsorbed on the surfaces of the plastic substrates is not treated and causes undesirable effects such as reduction in production yield and reliability.
The plastic substrate is composed of a plastic plate or film of acrylic, epoxy, polyethylene, or polycarbonate type, but if viewed microscopically, these materials have a sponge-like structure with an adsorptive property for gas and water. Consequently, if gas (including water vapor) adsorbed on the surface of the plastic substrate is released by a heating treatment in the production processes, it brings about such disadvantages as changed conditions for subsequent treatments and the development of bubbles.
For example, when the transparent electrode is to be formed on the plastic substrate by sputtering or vapor deposition in the electrode forming treatment, it is necessary to heat the substrate to a high temperature in a high-vacuum atmosphere. In this case, however, it is extremely difficult to obtain a high vacuum, compared with the case in which a glass substrate is used, so that a transparent electrode having an excellent strength and adherence to the substrate cannot be formed. The resulting transparent electrode is disadvantageous in that cracks, peeling off the substrate, flaws and breakage of wires are easy to occur therein.
The reason for not attaining a high vacuum can be attributed to the fact that gas and water (water vapor), which were adsorbed on the surface of the plastic substrate in a wet washing or like process, are released in a vacuum atmosphere by the application of heat.
Through an etching process prior to a seal printing treatment in which a sealant is printed on the electrode substrates and a washing process after the treatment of forming orientation films, the surfaces of the plastic substrates have adsorbed a plenty of water and gas, which in turn are released in the sealant by a thermosetting treatment (the application of pressure and heat) conducted after the sealant is printed and attached onto the plastic substrate, resulting in bubbles in the sealant. These bubbles in the sealant reduce not only the sealing strength but also airtightness of the sealant by causing bulk destruction of the sealant. As a result, bubbles are also mixed in the layer of a liquid crystal when the liquid crystal is injected by a vacuum impregnating injection method, thereby significantly reducing the reliability and weatherproofness of the device.
Moreover, while the evacuation conducted in the injection of a liquid crystal provides a sufficient degassing effect with a glass substrate, it does not provide a sufficient degassing effect with a plastic substrate, resulting in the undesirable development of bubbles after the injection of the liquid crystal. With a large-scale liquid crystal display device in the order of 10 cm by 15 cm, in particular, bubbles are always produced in the liquid crystal layer.
The object of the present invention, which was conducted in view of the above-mentioned circumstances, is to provide a method of producing a liquid crystal display device with which the reduction in production yield and reliability resulting from the gas adsorbed on the surfaces of the plastic substrates can be prevented.